12 PROCEEDINGS OF THE AMERICAN ACADEMY 



passes the equator, when the length of the embryo is about four fifths 

 of the diameter of the ovum. Tliey increase in number, irrow hu'fer. 

 coalesce by degrees, and finally blend in a single bubble-like vesicle in 

 the course of five hours. This vesicle, .01 mm. or more in diameter, 

 more than doubles its diameter in the next hour and a half; and, 

 steadily expanding, attains its maxinfum dimensions by the time the 

 blastopore closes. It is somewhat variable in size and shape, but 

 seldom more than .03 mm. in diameter, which is less than one tenth 

 the diameter of the caudal vesicle of the clasmobranch. Duriiis all 

 this time it lies beneath the chorda and the entodermic stratum, and 

 has no sort of relation with any tubular structure whatever. As the 

 alimentary canal is not yet in existence, it is difficult to see how this 

 vesicle can be the homologue of a dilatation which arises in, and has 

 no existence outside of, the post-anal gut. Ventrally and laterally it 

 is bounded by periblastic material, but it has no cellular envelope in 

 the strict sense of the word. 



Soon after the closure of the blastopore, the vesicle begins to grow 

 smaller, completely disappearing in the course of six hours. It is 

 during this waning period that it steps into relation with the posterior 

 end of the entodermic tract, which as yet has no lumen in any part. 

 We have followed this portion of its history several times in different 

 species of ova, and have satisfied ourselves that it is everywhere the 

 same. At the beginning of the wane, about one third of its surface 

 may be said to be enveloped by the entodermic stratum. At this 

 time this portion of its surface (upper hemisphere) is considerably 

 flattened, so that the entodermic envelope is really a very shallow 

 umbrella-like concavity. Its vertical diameter now lengthens, while 

 its horizontal diameter shortens ; at the same time the entodermic 

 concavity deepens, and its margin begins to form a plain constric- 

 tion around the equatorial zone of the vesicle. Gradually the lower 

 uncovered hemisphere rises up into the cavity, the whole vesicle grow- 

 ing rapidly smaller, until only a remnant remains, which is every- 

 where, except perhaps a small portion of its lower pole, enveloped by 

 the entodermic layer. This remnant of the vesicle keeps on dimin- 

 ishing in volume, without any important change in shape, until it 

 finally vanishes altogether. We are not able to say with certainty 

 that a lumen exists from this time onward in this portion of the ali- 

 mentary tract, but think this probable. 



The closing history of the vesicle has thus some analogy with that 

 of the caudal vesicle of the elasraobranch ; but we require more evi- 

 dence before concluding that they are hotnologous structures. 



